Andor said:
If it's a rare fancy piece of luxury jewlery you don't go to a dealer. You go to Christie's or Sotheby's and get full market price and the Auction house takes a 12-25% premium that gets paid by the buyer on top of what he's paying to you. Both Christie's and Sotheby's opened in about the 1770s btw, so this isn't exactly a new idea, nor is it dependant on modern telecommunications. Heck that's before the age of rail.
You're also assuming that Christie's and Sotheby's charged the same for their service
then as they do
now. And, while that's possible, I certainly wouldn't take it as gospel without research.
And, as has been pointed out, getting your item to them for auction is, in that timeframe, an adventure in and of itself. It's not like you could ship it UPS in 1770. You'd either take it there yourself or use a private, respectable (and very well-paid!) courier.
Not to mention that 1770 is pretty solidly
past most of the historical models most of us use for D&D games. Although an interesting game could no doubt be set in that period, it would involve changing the social assumptions of the assumed D&D setting a LOT.
Instead of the age of Englightenment, think of Elizabethan England. Your adventurers are the equivalent of Francis Drake and his ilk. Your magic sword is like a captured merchant vessel. Who do they sell something like that to? The only people who can afford it are some nobles, the Queen and the richest merchanting guilds. If they're not willing to pay your price, you can either a) take the price they do offer - no doubt a pittance of full value, or b) hold on to your thing until someone offers you a better price.
In period, both were common. However, the difference is that a pirate like Drake actually has a use for two ships. How much use does your fighter actually have for a backup magic sword?
I imagine giving them as gifts to people you're trying to curry favor with or selling them at a steep discount (or even selling at a steep discount in an attempt to curry favor) are all a lot more common than getting full price.